In the combustion process of a gas turbine engine, nitrogen oxides and other types of regulated emissions are produced. One solution for reducing the overall levels of nitrogen oxide emissions is the use of a selective catalyst reduction system. Generally described, the selective catalyst reduction system adds a reductant, typically ammonia or urea, to the combustion gas stream before passing the stream through a catalyst bed so as to absorb selectively the nitrogen oxides and the reducing agent. The absorbed components undergo a chemical reaction on the catalyst surface and the reaction products are desorbed. Specifically, the reactant reacts with the nitrogen oxides in the combustion gas stream to form water and nitrogen. Other types of catalysts and other types of reductants may be used.
The overall efficiency of the selective catalyst reduction system may depend in part on the temperature of the combustion gas stream. Specifically, the efficient temperature range of the selective catalyst reduction system may be relatively narrow. As such, the hot combustion gas stream generally should be cooled before reaching the catalyst. Moreover, the gas stream should have a substantially even temperature profile before reaching the catalyst.